While normally dormant, a Fortress will occasionally appear with a large skull cloud hovering above it, visible from a distance. This will happen approximately three hours after the last raid on the same server has been completed, however sometimes this will occur much earlier than that. When this cloud is present, the Fortress is active with waves of Skeletons manning the island's Cannons.
There are roughly 10-15 waves per raid.
At the final wave, the Fort's Skeleton Lord will spawn. It will drop a Vault Key upon death and also cause the other remaining Skeletons to fade away, the Vault Key can be used to unlock a Fortress Vault that contains a large amount of loot.
A skilled sailor can stop their ship on the south side of the Fort to not get shot at.